Electrical circuit breakers are used in electrical distribution systems to protect electrical loads and conductors from being exposed to overcurrent conditions. In general, there are two types of overcurrent conditions—an overload and a fault. The National Electrical Code (NEC) defines an overload as: “operation of equipment in excess of normal, full-load rating, or conductor in excess of rated ampacity that when it persists for a sufficient length of time, would cause damage or dangerous overheating.” Faults typically produce much higher overcurrents than do overloads, depending on the fault impedance. A fault with no impedance is referred to as a “short circuit” or as a “bolted fault.”
Conventional circuit breakers are mechanical in nature. They have electrical contacts that are physically separated upon the occurrence of a fault or prolonged overcurrent condition. Opening the contacts is normally performed electromagnetically, using a spring mechanism, compressed air, or a combination of a spring mechanism and compressed air.
One significant problem with conventional circuit breakers is that they are slow to react to faults. Due to their electromechanical construction, conventional circuit breakers will typically require at least several milliseconds to isolate a fault. The slow reaction time is undesirable since it raises the risk of fire hazards, damage to electrical equipment, and even arc flashes, which can occur when a short circuit or bolted fault is not isolated quickly enough. An arc flash is an electrical explosion of the electrical conductors that create the short-circuit condition. The energy released in an arc flash can produce temperatures exceeding 35,000° F. (or 20,000° C.) at the arc terminals, resulting in rapidly vaporizing metal conductors, blasting molten metal, as well as expanding plasma that is ejected outwards from the point of incident with extreme force. Arc flashes are therefore clearly extremely hazardous to life, property and electrical equipment.
In addition to being slow at isolating faults, conventional circuit breakers are highly variable. Due to limitations on the magnetics and mechanical design involved, the time it takes, and the current limit at which, a mechanical circuit breaker trips in response to a fault or prolonged overcurrent condition can vary in a single given circuit breaker and can also vary from one circuit breaker to another, even for circuit breakers that are of the same type and same rating, and even of the same type and rating from the same manufacturer.
Due to the lack of precision and high degree of variability of conventional circuit breakers, manufacturers will typically provide time-current characteristic data for each type and rating of circuit breaker that they manufacture and sell. The time-current characteristic data for a particular type and rating is often displayed in a two-dimensional logarithmic plot, such as illustrated in FIG. 1, with current on the horizontal axis, time on the vertical axis, time-current regions in which the circuit breaker is guaranteed to trip and not trip, and uncertainty bands within which the trip status of the breaker is uncertain.
The lack of precision and high degree of variability of conventional circuit breakers make coordination studies difficult to perform. A coordination study is a study performed by an electrician or engineer during the design of an electrical distribution system. The coordination study involves selecting circuit breakers, often of different ratings, and figuring out the best way to arrange the various selected circuit breakers in the electrical distribution system. One important task involved in the coordination study involves configuring the various circuit breakers in such a way that only the closest circuit breaker upstream from an impending fault will trip to electrically isolate the impending fault. If this task is properly performed, the time-current uncertainty bands of the various circuit breakers will not overlap. Unfortunately, due to the lack of precision and the resulting uncertainty bands in the time-current characteristics, the coordination study cannot always be completed as required or desired, and the uncertainty bands of the various circuit breakers end up overlapping to some extent, as illustrated in FIG. 2. The overlapping uncertainty bands is problematic since it results in the possibility that upstream circuit breakers will trip prematurely or unnecessarily in response to an impending fault, instead of by a circuit breaker located further downstream that is closer to the impending fault. The premature or unnecessary tripping of the upstream circuit breaker is undesirable since it can result in a larger section of the distribution system being de-energized than is necessary.
Conventional circuit breakers provide high isolation capability, once they have been tripped. However, their slow reaction times, lack of precision, and high degree of variability are all undesirable characteristics. Not only do the slow reaction times result in inadequate protection against the possibility of arc flashes, the high degree of variability and lack of precision make coordination studies difficult, and in some cases even impossible, to perform. It would be desirable, therefore, to have a circuit breaker that has the high isolation capability offered by conventional electromechanical circuit breakers but which also has the ability to react to and isolate faults and other overcurrent conditions much more rapidly than conventional electromechanical circuit breakers are capable of. It would also be desirable to have a fast-reacting circuit breaker that has time-current characteristics that are precise and which can be programmed, even dynamically and in real time, with a high degree of precision.